Well, I just tested it and the Wiki seems to be wrong, he takes equal damage in the air and the ground. That helps him go from a Heavy spoiler to a Heavy tank who can spoil.
The paralyze rate is essentially 100%. It hits five times and it seems to cause paralysis roughly one every three hits. So, unless I'm missing something, altogether the attack has a ~155% chance of paralysis.
I'm sure it unlocks at 55% HP. The yellow bar of HP is the first 90% of a boss' HP (unless a reliable source sayse differently), and he unlocks the extra moves once half his yellow bar is gone.
As for most of his attacks not hitting, I couldn't seem to get away from his claw anywhere unless I used the invincibility frames from dashing (or by going invisible), and the thunder bolts hit you anywhere unless you're dashing for invincibility frames. I'm pretty sure the shockwave his punch causes hits nearly anywhere too. So all his bottom screen attacks that matter will hit. Plus, if you hide in the corner, his thunder balls of doom are much more likely to hit (multiple times) which can only help his damage.